1. Field of the Invention
This invention relates to a laying structure for a vacuum sewer pipe of a vacuum sewage collecting system for collecting sewage discharged from homes and facilities.
2. Prior Art
A vacuum sewage collecting system is widely used to collect sewage discharged from homes and facilities.
FIG. 3 is a view representing a general construction of a vacuum sewage collecting system of this kind.
As illustrated therein, sewage discharged from each home 30 flows into a cesspool 32 located underground by way of an underground sewer pipe 31 under the influence of gravity. When the sewage accumulates at a lower portion of the cesspool 32 in a predetermined quantity, a vacuum valve 33 mounted on an upper portion of the cesspool 32 opens, and the sewage in the cesspool 32 is sucked in from a suction pipe 34.
The sewage is then sucked into a vacuum sewer pipe 1 laid like network in the ground by way of the vacuum valve 33 and collected in a sewage tank 41 in a vacuum pump house 40.
The sewage accumulated in the sewage tank 41 is then sent to a sewage treatment plant and so forth by a pressure feed pump 42. To maintain both the sewage tank 41 and the vacuum sewer pipe 1 at an internal negative pressure, a vacuum pump 43 is connected to the sewage tank 41.
FIG. 4 is a side sectional view representing a state wherein the vacuum sewer pipe 1 is buried in the ground in a plain topography.
As illustrated therein, the vacuum sewer pipe 1 is usally laid so that it includes a downward pitch portion 11 having a slope of 0.2 to 0.3% with reference to the horizontal plane toward the downstream side (that is, on the sewage tank 41 side in the vacuum pump house 40), and when it is deepened by a predetermined depth from an original or first laying level by the downward pitch portion 11, a short upward pitch portion 12 is provide so as to return the laying depth of the pipe to the original laying level.
In such a system, as shown in FIG. 5, even if the sewage having flowed on the downward pitch portion 11 as being drawn by the air accumulates at the deepest portion, since the air and the sewage are successively advanced into the deepest portion, the sewage accumulated in this portion is blown up by the air to exceed the upward pitch portion 12, and thus flows further ahead.
Meanwhile, a depth of the connecting portion between the upward pitch portion 12 and the downward pitch portion 11 was selected hitherto at a position considerably deeper than the depth corresponding to a bore of the vacuum sewer pipe 1 from the original or shallowest laying level of the vacuum sewer pipe 1.
That is, for example, a height of the upward pitch portion 12 was selected at 200 mm or 300 mm in the case of vacuum sewer pipe 1 100 mm in bore.
However, such a pipe laying structure caused the following problems.
(1) Where the quantity of air flowing in the vacuum sewer pipe 1 is low, an air lock may be formed at a portion of the upward pitch portion 12 as shown in FIG. 6, thus a degree of vacuum on the upstream side is abated according as it goes toward the end of a line of the vacuum sewer pipe 1.
(2) If the vacuum sewer pipe 1 is laid with the downward slope of 0.2 to 0.3% in plain ground, a laying depth gradually increases. However, in the aforementioned prior art system, since the laying depth is returned to the original laying depth by providing an upward pitch portion at a spot considerably deeper than a level of the minimum laying depth required for roads, the laying depth greater than necessary on the whole, thus leading to an increase in construction costs.